1. Field of the Invention
The present invention relates to a plating apparatus and method for performing plating on a long conductive substrate.
2. Description of the Related Art
Plating apparatuses and methods for continuously performing plating on a long conductive substrate are used in various technical fields. For example, photovoltaic devices, such as solar cells or the like, are configured by laminating a reflective layer, a transparent layer (for example, a zinc-oxide layer), a semiconductor layer and a transparent conductive layer on a supporting member. Plating apparatuses and methods for a long substrate are sometimes used for manufacturing such devices.
The configuration of a photovoltaic device, and a method for manufacturing the same, and the like will now be described.
Hydrogenated amorphous silicon, hydrogenated amorphous silicon germanium, hydrogenated silicon carbide, microcrystalline silicon, polycrystalline silicon or the like is used for the semiconductor layer.
The reflective layer has the function of improving absorption efficiency for long-wavelength light, and desirably has a reflection property effective at wavelengths near the band gap of a semiconductor material where absorption is small, i.e., wavelengths of 800–1,200 nm. A metal layer made of gold, silver, copper, aluminum or the like sufficiently satisfies this condition.
The transparent layer (for example, a zinc-oxide layer) is disposed between the reflective layer and the semiconductor layer for confining light and improving a short-current density Jsc by effectively utilizing the reflective layer. In order to prevent degradation of characteristics due to a shunt path, a layer made of a conductive transparent material, i.e., a transparent conductive layer, is provided between the reflective layer and the semiconductor layer. Usually, these layers are deposited according to vacuum deposition or sputtering, and improvement in terms of a short-current density equal or more than 1 mA/cm2 is obtained.
For example, in “A light confinement effect in an a-SiGe solar cell on a 29 p-MF-22 stainless steel substrate”, Extended Abstracts (the 51st Autumn Meeting, 1990) of the Japan Society of Applied Physics, p. 747, or in “P-IA-15a-SiC/a-Si/a-SiGe Multi-Band Gap Stacked Solar Cells with Band Gap Profiling”, Sannomiya et al., Technical Digest of the International PVSEC-5, Kyoto, Japan, p. 381, 1990, studies have been done on the reflectivity and the texture structure of a reflective layer made of silver atoms. In these examples, effective projections and recesses are formed by using two silver layers deposited at different substrate temperatures as the reflective layer, and an increase in short current due to a light confining effect is achieved by combination with a zinc oxide layer formed thereon.
The transparent layer used as the light confining layer is deposited according to vacuum deposition by resistance heating or electron-beam heating, sputtering, ion plating, CVD (chemical vapor deposition) or the like. However, a high cost of vacuum deposition apparatuses, a high manufacturing cost of target materials, inferior efficiency of utilization of materials, and the like result in a very high manufacturing cost of photovoltaic devices using these techniques, and cause large problems for industrial application of solar cells.
As a method for solving these problems, a technique for manufacturing zinc oxide according to liquid deposition (“Formation of a ZnO film according to aqueous electrolysis”, Extended Abstracts (the 65th Autumn Meeting, 1995) of the Japan Society of Applied Physics) has been reported.
In Japanese Patent Application Laid-Open (Kokai) No. 10-195693 (1998) (U.S. Pat. No. 5,804,466), a method for forming a zinc-oxide layer according to electrolytic deposition (identical to electroplating, hereinafter abbreviated sometimes as “ED”) has been proposed. In this publication, a method for forming a zinc-oxide layer on a conductive substrate by immersing a conductive substrate and an electrode in an aqueous solution containing nitrate ions, zinc ions and carbohydrate, and applying a voltage between the conductive substrate and the electrode.
In Japanese Patent Application Laid-Open (Kokai) No. 10-140373 (1998) (U.S. Pat. No. 5,804,466), a method for forming a uniform zinc-oxide layer having an excellent substrate adhesion property according to ED has been proposed. More specifically, the method has a process of forming a first zinc-oxide layer on a substrate according to sputtering, and a process of forming a second zinc-oxide layer on the first zinc-oxide layer by immersing the substrate in an aqueous solution containing at least nitride ions, zinc ion and carbohydrate, and applying a voltage between the substrate and an electrode immersed on the solution.
According to these methods, since a vacuum deposition apparatus and a target that are expensive are unnecessary, it is possible to greatly reduce the production cost of zinc oxide. Since deposition can also be performed on a large substrate, these methods are promising for manufacturing a large photovoltaic device, such as a solar cell.
However, such methods of electrochemically depositing zinc oxide have problems to be solved.
That is, when forming a zinc-oxide layer according to ED, if a conductive substrate is used, a zinc-oxide layer is more or less deposited also on a non-film-forming surface (the back) as well on a film-forming surface of the substrate. The zinc-oxide layer deposited on the back of the substrate (hereinafter termed a “back film”) sometimes has a property different from the property of the zinc-oxide layer deposited on the surface of the substrate depending on deposition conditions (mainly a manner of application of an electric field). More specifically, the back film sometimes becomes a low-density and fragile film having different surface roughness and mechanical strength. If such a back surface is more or less present, the following problems will arise when forming, for example, a semiconductor device, such as a photovoltaic device (solar cell) or the like.
(1) When using a substrate having a zinc-oxide layer formed thereon according to ED for manufacturing a photovoltaic device, photoelectric conversion characteristics may be degraded due to degassing in a vacuum apparatus. Particularly, since the back film tends to have a low density and a large surface area, there is a large possibility of introducing absorbed gases, such as oxygen, nitrogen, water and the like, into the vacuum apparatus.
(2) When conveying the substrate within the vacuum apparatus, the back film may peel to produce dust, resulting in contamination of the inside of the vacuum apparatus, and degradation of characteristics by being mixed in the semiconductor layer or the like.
(3) When a roll-to-roll configuration is used, the back film is also wound during a winding process, in which the back surface may peel to be mixed between substrates as foreign matter. In this case, the foreign matter may contact the zinc-oxide layer deposited on the surface, thereby causing a possibility of producing damage.
(4) Deviation in winding and problems in conveyance may occur due to variations in the coefficient of friction caused by the presence of the back film.
(5) When performing soldering, bonding by an adhesive, or the like at the back surface as a post-process after forming the zinc-oxide layer, the film of the back surface may cause insufficient soldering, degradation of the adhesive property, and the like.
In other technical fields, when intending to perform plating only on one surface of a long conductive substrate, the presence of the back film may cause various problems, such as bad influence on a post-process, degradation of appearance, and the like. Accordingly, a method for preventing formation of the back film as much as possible, or removing the formed back film is requested.
In order to remove the back film, Japanese Patent Application Laid-Open No. 11-286799 (1999) discloses a method of etching a zinc-oxide layer deposited on the back of a substrate according to electrolysis using a back-film-adhesion preventing electrode. This method can greatly reduce formation of the back film. However, it is difficult to remove only a zinc-oxide layer deposited on the back of a substrate without badly influencing a zinc-oxide layer formed on the surface. Furthermore, when using a metal film that is reactive with an oxidizing liquid, such as silver or the like, between the substrate and zinc oxide, there is the possibility that electrochemical reaction is also exerted to the metal film by an electric field for etching, and problems, such as discoloration, dissolution and the like, may arise in the metal film.
Japanese Patent Application Laid-Open (Kokai) No. 10-60686 (1998) (application in a second country not confirmed) discloses a technique for preventing adhesion of plating on a non-plating surface by disposing, when performing continuous plating on one surface of a metal strip, an insulator, operating as a blocking member, between an edge portion of the strip and an anode. A similar blocking technique is also described in Japanese Patent Application Laid-Open (Kokai) No. 2002-155396 (2002) (application in a second country not confirmed). However, these techniques have the problem that it is very difficult to perform optimum design of an apparatus for effectively preventing adhesion of plating on a non-plating surface as well as performing uniform plating on a plating surface. Furthermore, since such optimum design may differ depending on plating conditions, it is impossible to design an apparatus capable of flexibly dealing with changes in plating conditions.
Japanese Patent Application Laid-Open (Kokai) No. 10-259496 (1998) (U.S. Pat. No. 6,077,411) proposes a technique for preventing deposition of a zinc-oxide layer on the back of a substrate when forming a zinc-oxide layer according to ED. More specifically, deposition of an unnecessary zinc-oxide layer on the back of a long substrate immersed in an aqueous solution containing nitrate ions and zinc ions by providing a rotating belt for conveying the substrate while covering one surface of the substrate.
According to this method, it is possible to effectively suppress deposition of the back film. However, since it is necessary to provide a configuration of conveying a member for covering the back of a substrate, the configuration of the apparatus is complicated, and the cost of the apparatus will increase.